1. Field of the Invention
This invention relates to a new and improved magnetic damping system for producing magnetic braking or retarding torque on a conductive disk of an induction device, and more particularly, to a damping system including a pair of highly coercive and anisotropic permanent magnets.
2. Description of the Prior Art
Magnetic braking for eddy current brake arrangements is commonly included in induction-type meter and relay devices, and in particular, in induction electromechanical watthour meters. An electromagnetic unit having voltage and current sections is typically included in induction watthour meters for connection between an electrical source and a load for measuring the consumption of AC electric energy. AC magnetic fields from the electromagnetic unit produce a driving torque on a rotatable armature formed by an electroconductive disk. Interaction between the changing magnetic fields and changing eddy currents induced in the disk by the fields develops a driving torque on the disk. Associated with the electroconductive disk is a permanent magnet brake or damping system for directing a magnetic field into the disk which induces eddy currents. The braking magnetic field reacts with the eddy currents to produce a retarding torque on the disk. The retarding torque is proportional to the disk speed for a given magnetic field and balances the disk driving torque. Thus, the disk speed is maintained accurately proportional to the electric power applied through the meter so that each disk rotation is representative of a predetermined quantum of electric energy consumption.
The braking magnetic flux is typically provided by permanent magnets which direct a flux through an air gap in which the electroconductive disk rotates. The strength of the permanent magnets, the position of the braking magnetic flux in relation to the disk center, the area and density of the braking flux entering the disk, and the length of the air gap determine the amount and consistency of the retarding torque. Maintaining the braking magnetic flux constant is a chief requirement. It is essential that the retarding torque that is produced by the damping assembly be kept proportional to the speed of the meter disk for accurate meter measurements. This becomes increasingly difficult due to the fact that the meter is exposed to widely-varying temperature and atmospheric changes. The meter is subjected to strong demagnetizing effects caused by electrical surges. Finally, substantial mechanical shock and vibration often occurs in shipping and handling of the meter.
The design of a magnetic damping system must incorporate the above and other factors in an assembly that is easy and simple to manufacture at minimum cost in accordance with high volume production techniques. Various magnetic damping systems are currently available which utilize a wide variety of configurations and various types of magnetic materials.
One such magnetic damping system is disclosed in U.S. Pat. No. 4,238,729. A high coercive magnet is vertically elongated and has a horizontal direction of magnetization which is parallel to the electroconductive disk. Flux concentrating pole pieces adjoin the magnetic pole faces for vertically directing two closely spaced braking magnetic fields from a pair of pole tips into the disk. Temperature compensation is provided by covering the sides of the permanent magnet between the edges of the pole pieces.
U.S. Pat. No. 4,182,984 is for a watthour meter damping assembly including a U-shaped magnetic yoke and a pair of highly-coercive and anisotropic permanent magnets projecting in a facing relationship from the ends of the yoke. The opposite pole faces of the magnets form an air gap in which the meter disk rotates. Temperature compensation is provided by a temperature compensator which extends over the pole faces of the permanent magnets to form a compensating shunt flux path around the air gap.
German Patentschrift No. 804,694 illustrates various methods using as many as four individual permanent magnets in a retaining frame which provides return flux paths. The retaining frame is constructed so as to expand and contract thereby varying the air gap to compensate for changes in the permanent magnets due to temperature variations.
Finally, U.S. Pat. No. 4,030,031 illustrates a magnetic damping system utilizing two permanent magnets, two magnetic bridge pieces, and two flux return paths to provide a substantially square magnetic flux path.